1. Field of the Invention
This invention relates generally to a prosthesis mounting device for securing a prosthesis on an implant supported in bone tissue.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Conventional crowns, bridges and dentures have long been the standard prosthetic devices used to replace missing teeth. Such devices are secured to a jaw bone using a surgical anchor known as a dental implant or a prosthesis mounting device.
The design of dental implants has developed considerably over the years since their initial conception by the ancient Egyptians. One of the more important developments is credited to the Swedish firm, Noblepharma. In the mid-1980s, Noblepharma developed and marketed a dental implant that resembled a natural tooth root. This implant, known as a root form implant, include two major components: a bone screw and a prosthetic abutment interface. There are now about 10 major dental implant manufacturers in the United States alone that market various forms of the root form implant.
The protocol for surgical placement of a root form implant is to first expose an underlying jaw bone through a surgical intervention of the soft tissue covering the jaw bone. An opening is then created in the jaw bone equal to the minor diameter of the bone screw portion of the implant. This is done by using a series of expanding diameter surgical drills that allow the surgeon to gradually increase the size of the hole in the bone until the implant can be screwed into place. Once the implant has been screwed in, the soft tissue incision is sutured closed over the osteotomy site. In a process known in the art as osseointegration, the bone tissue around the device then grows into tight apposition to the screw during a three to six month healing process.
In recent years, however, some doctors have advocated what is known in the art as immediate or progressive loading of an implant. This process eliminates or shortens the time required for osseointegration and its associated long healing times. However, most implants still require that patients wait for an extended time before they can fully make use of permanent replacement teeth. Multiple surgeries are also required.
After a root form screw implant is firmly positioned in bone, and once the osseointegration process is complete, the tissue covering the osteotomy site is again reflected and an attachment device called an abutment is affixed to a head of the implant that remains exposed after installation. A dental prosthetic can then be cemented or screwed onto the prosthetic abutment portion of the implant. The prosthetic abutment portion of the device is available in many shapes, sizes and designs to accommodate various treatment applications.
Extending healing periods and costs associated with the above described two-stage surgical implant procedure have prevented traditional dental implant systems from becoming the chosen treatment modality for patients with missing teeth. Manufacturers and marketers of dental implants have been searching for new concepts and ideas that would provide a more economical and less surgically invasive system.
In response to this need, a New York City company called Dentatus® USA Ltd. began experimenting with endodontic posts designed to be implanted directly into a jaw bone. These endontic posts, known as MTI implants, had a one-piece design incorporating both screw and abutment. Because the diameter of the MTI implant was only 1.8 mm, Dentatus® was able to develop an implantation procedure that did not require a large opening in the bone to receive the implant. All that was necessary was a small, shallow starter hole that could be formed in jaw bone tissue directly through soft tissue without having to surgically lay back a flap of the soft tissue to expose the bone beneath. This new approach was minimally invasive and provided an implant that could be immediately loaded without having to wait for an extended period of time for osseointegration to occur. In the art, this new type of implant that's dimensionally small enough to self-tap into bone tissue without splitting the bone tissue became known as the mini dental implant.
While some standard sized (approx. 3.75 mm diameter) implants claim to be self-tapping, because of their larger size, the extent of their self-tapping is severely limited. Bone has a visco-elastic nature that allows it stretch, to a certain point, to accommodate inserted objects. However, to install an implant shaft larger than approximately 2.0 mm in diameter requires a large osteotomy formed by drilling progressively larger osteotomy holes to the full depth that the implant will extend into the bone. For example, a so-called self-tapping implant having a 3.75 mm diameter and 4.0 mm diameter threads will still require a 3.0 mm osteotomy. A cutter is supported near the tip of such an implant and extends radially outward to engage and form thread grooves in the wall of a 3.75 mm osteotomy as the implant is installed.
IMTEC® Corporation is currently marketing a Sendax mini dental implant system that comprises a prosthesis mounting device having a threaded shaft, and an abutment including a square nut and a ball-shaped O-ring abutment. The threaded shaft is tapered at a first end to allow the shaft to self-tap into bone tissue starting from a small, shallow pilot hole formed in bone tissue. Because the shaft self-taps past the depth of the pilot hole, it immediately integrates with the bone tissue. The square nut is attached to and extends integrally and axially from a second end of the threaded shaft opposite the first end. The O-ring abutment is attached to and extends integrally and axially from the square abutment. The O-ring abutment is shaped to engage and support a prosthetic tooth or set of teeth. However, this system is unable to disconnect or automatically discontinue torque application during installation when a predetermined bone density is encountered. Nor can such a device warn an installer that the bone tissue lacks sufficient density to properly support a prosthetic tooth. Still further, the Sendax mounting system cannot indicate to an installer when it is fully seated in bone tissue, the platform formed at the head area of the shaft is no greater than the cross-sectional area of the shaft itself and provides little support for a prosthesis, and it doesn't provide a satisfactory interface between the implant and surrounding soft tissues.
In implant dentistry, it is also known for a prosthesis mounting assembly to include a large titanium appendage or “preppable” abutment that detachably extends from an axial top surface of a full-sized dental implant or “tooth post”. Such a preppable abutment is milled or “prepped”, as a tooth would be prepped, into a generally triangular prism-like shape suitable to accept and support a crown or bridge. It's then fixed to the top surface of an implant using an axially-oriented prosthetic fixation screw. Typically, a preppable abutment of this type will also include either an internal or an external hex key or recess that engages a complementary recess or key formed on or in the axial top surface of an implant and the appendage may be milled either in the mouth of a model using a dentist's drill or by sending the preppable abutment to dental lab where a milling machine is used to prep the appendage.
In practice, an implant is first surgically installed by incising and laying back gum tissue to reveal jaw bone tissue, drilling a hole in the bone, inserting the implant, then closing the gum tissue over the osteotomy site and allowing oseointegration to occur. The preppable abutment is then installed on the implant by re-incising and laying back the gum tissue to reveal the axial top surface of the implant, positioning the preppable abutment on the implant, and fixing it in place with the prosthetic fixation screw. The gum tissue is then closed and allowed to heal. An impression is then taken of the preppable abutment and a coping is formed in the impression to duplicate the preppable abutment. The coping is used to form a stone model duplicating the patient's mouth. A prosthetic tooth or bridge is then formed on the stone model and coping to fit in the patient's mouth and over the preppable abutment extending upward from the embedded implant. Finally, the prosthetic tooth or bridge is supported on the preppable abutment. However, this type of preppable abutment requires an implant that is specifically configured to accept and support it. IN addition, this design is prone to micro movement that can but stress on and eventually break a prosthetic tooth or bridge.
What's needed is a prosthesis mounting device and assembly that doesn't require an implant that's specifically designed to support it.